The present invention relates to a reversible refrigeration system or heat pump. Many current heat pump systems use a four-way, two-position valve which is reversed in order to defrost the system. Such conventional hot gas defrost causes undesirable strong pressure changes and associated oil forming at the compressor. In addition, the four-way, two position valves used in such systems are complex and expensive and have several disadvantages.
One disadvantage of a four-way, two-position or reversing valve is that heat is lost by condition and leakage from the hot and the cold refrigerant lines within the valve. U.S. Pat. No. 2,991,631 recognizes this problem. For example, in column 1, at lines 34-40, the patent notes that "Another important feature of the present invention is that there is a minimum amount of heat transfer between various conduits in the four-way valve to minimize that heat transfer which otherwise would be a dead load on the refrigeration compression system . . .". This heat loss or "dead load" problem can significantly decrease the efficiency of a heat pump system. The present invention substantially eliminates this problem by eliminating the four-way, two-position reversing valve.
Another long standing problem with four-way, two-position valves is leakage of working fluid from the high to low pressure side of the valve. U.S. Pat. No. 2,741,264, line 32 et seq. documents this problem. The present invention employs simpler valves which are more likely to provide leak-free operation.
Four-way, two-position reversing valves also typically have problems associated with unequal thermal expansion of metal parts within the valve, a problem related to the first two problems discussed above. The temperature differential between the suction and discharge lines of the system compressor causes thermal stresses in the four-way valve. That is, the cold portions of the valve tend to expand less (or contract more) than the hot portions. This unequal expansion causes sealing problems and contributes to binding of the moving parts within the valve. U.S. Pat. No. 4,055,056, column 1, line 27, discusses this problem. The valves of the present system do not simultaneously have cold and hot gasses flowing through them and, accordingly, do not undergo thermal straining to as large a degree as the four-way, two-position valves typically used in current systems.
In addition to the above advantages, the present invention provides for a condensor bypass defrost, again without the use of a four-way, two-position reversing valve. Condensor bypass defrost is more reliable than full reverse defrost typically used in four-way valve systems since compressor operating conditions change less abruptly during condensor bypass defrost. Further, in condensor bypass defrost, the first in-rush of hot refrigerant is used to defrost the evaporator rather than to warm up the accumulator, thus increasing defrost efficiency (the accumulator, not shown in FIG. 1, is located at the compressor suction port).
The present invention also provides a stop mode of operation which increases system efficiency by preventing hot, liquid refrigerant from escaping from the evaporator immediately following compressor shut down at the end of a cycle.